Tumor and brain hypoxia will be probed using multiple measurement techniques: EPR oximetry, diffusion weighted MRI, Gd-DTPA kinetic perfusion measurements, P31 NMR spectroscopy, and for the brain measurements, doppler flow measurements. Correlates between the techniques have already been demonstrated in rat brain and mouse tumors. The hypothesis that there is a set point in terms of pO2 below which profound physiologic changes take place that can be pharmacologically modified will be tested.